A New Microcrack Characterisation Method for Quench Cracking in Induction-Hardened Steels

High-performance induction-hardened bearing steels are prone to quench cracking during manufacturing, causing significant material and energy waste. Understanding the physics behind microcracking is essential to the design of alloys and processes with reduced cracking behaviour. However, conventional quench crack analysis methods provide information only on crack severity and neither link martensite microstructure to microcrack formation or provide meaningful insights on the origins of microcracking. Therefore, this work introduces a new crack quantification method that assesses various crack features, including crack length, location within a martensite plate, crack angle to the plate midrib, and the distance from the induction-hardened surface. It is found that microcrack severity changes with the distance from the induction-hardened surface, peaking at ∼1 mm in depth, with a maximum density of approximately 1000 cracks per mm2. In addition, microcracks are mostly seen in the martensite plates rather than at the austenite–martensite interface, with the majority lying perpendicular to the midrib. Approximately 50% of the interfacial cracks are oriented at an angle less than 10 °C to the martensite midrib and are mainly located around the midpoint of the interface. Martensite plates having interfacial cracks are mostly 10–20 μm long, whereas martensite plates with internal cracks are mostly 20–30 μm long. The new method helps build quantitative links between microcracking and martensite morphology to study the mechanisms of cracking and the role of the initial microstructure in more detail.